Update member_slug lookup to 404 when not found

[CloCkWeRX]

Updated several controllers (Seeds, Activities, Posts, Plantings, Harvests, and Gardens) to use Member.find_by!(slug: params[:member_slug]) when params[:member_slug] is present. This ensures that an ActiveRecord::RecordNotFound exception is raised (leading to a 404 response) if an invalid slug is provided, while maintaining existing filtering behavior when a valid slug is found.

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PR created automatically by Jules for task 17599607196474746471 started by @CloCkWeRX

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[CloCkWeRX]

#1640